1. Field of the Invention
There are two main types of such members. The first type cut a thread in a hole into which they are driven with the production of swarf and chips. The second type, which is the type with which the present invention is concerned, form or roll a female thread in a plain hole into which they are driven by displacement of the material in which the hole is formed and without the production of swarf or chips. Such members will hereinafter be referred to as "screw-forming" members.
2. Description of Prior Art
Various designs of such screw-forming members are at present in use or have been described in patent literature.
One such design has a cylindrical threaded shank with a tapered free end portion to form the female thread and the forming torque required, i.e., the torque required to drive the member into the hole to form the thread, is comparatively high.
Another design differs from the foregoing in having a series of circumferentially spaced flats on the tapered free end portion, such flats providing relief portions and thus reducing the forming torque required as compared with said one design. With said design, since the flats are on the taper their effectiveness in providing relief decreases progressively as the member enters the hole and binding between the flanks of the male and female thread progressively increases. The flats interrupt the thread form on the tapered free end portion so that between each adjacent pair of flats there is a threaded rib. As the member progresses into the hole, the crest of a threadturn on the leading edge of one rib interferes with the root of the female thread formed by the preceding thread rib causing binding and the possibility of cutting and the production of swarf.
In a further design the tapered free end portion of the shank is of lobular cross section as distinct from being of circular cross section with or without flats. The free end portion of the shank is formed to lobular cross section before it is threaded. The lobular cross section provides lobes which form the female thread and relief portions between the lobes. In the relief portions the female thread is totally relieved, i.e., on root, flanks and crest as compared with said other design with flats where as the member enters the hole, the flats give progressively less relief on the root and flanks of the female thread. Such further design has a desirably low forming torque.
Between the tapered free end or forming portion such members have a portion of thread which is considered to be a holding portion which, when the member is tightened, distorts with the female thread to hold the member tight. Some members have this holding portion of lobular cross section and this limits the stripping strength of the member since the engagement and distortion of the male and female threads on tightening is limited to areas provided by the lobes of the male threads. It is desirable to provide a holding portion of cylindrical cross section so that the maximum area of engagement of the male and female threads is obtained on tightening.
Such members usually form a female thread wherein the radial distances between the longitudinal axis of the shank and the roots and crests of the thread are substantially the same as the similar radial distances of the roots and crests respectively of the thread on the holding portion of the member. Where the holding portion is cylindrical this will be so throughout the whole of each thread turn but where the holding portion is of lobular cross section it will only be true of the male thread on the lobes.
However, in either case there is binding between the male thread on the holding portion and the formed female thread and when driving the member into the hole it is necessary to apply torque to overcome this binding in addition to the torque required to form the thread. The further the member penetrates into the hole the greater will be the total torque required since the torque required to overcome the binding will increase. In practice it is necessary to limit the total torque and this is achieved by specifying, for a given member, a greater diameter of starting hole the greater the required depth of penetration of the member. Obviously the greater the diameter of the hole the more truncated is the female thread produced therein thus reducing the total torque by reducing both the forming torque and the torque to overcome binding. The more truncated the female thread the less is the stripping strength of the fastening provided by the male and female members. In practice the truncation of the female thread may reach 50%.
A screw-forming member has also been proposed which has at the free end of the shank a tapered portion and then a thread forming portion having a thread which is slightly larger than, but has the same root diameter as, the thread on the holding portion of the shank. This will give desirable clearance between the formed female thread and the thread on the holding portion. However, the thread forming portion is circular in cross-section and the thread forming torque will be high. Moreover it is necessary to provide extra metal at the thread forming portion of the shank to be able to provide at said portion a larger thread having the same root diameter as the thread on the holding portion and such a construction does not lend itself to modern production practice on solid-die, bolt-forging machines.
A screw-forming member of circular section has also been proposed which has a thread forming portion with a thread whose root and crest diameters are greater than the corresponding diameters of the thread on the holding portion. This member is intended for use in sheet metal and the thread-forming torque would be too high to use this member in deep holes.
Some designs of member have a change of core section from lobular to cylindrical between the thread forming and holding portions and some have a change of thread size between said portions. With such designs non-standard and expensive thread rolling dies are necessary and a different die or a different combination of dies is necessary for each thread length required.